The present invention relates to power-assisted steering systems for wheeled vehicles, and more particularly to a changeover valve unit for controlling the supply of fluid under pressure to a hydraulic cylinder associated with the steering system.
In general, such a conventional changeover valve unit as described above comprises input and output shafts arranged for relative rotation on aligned axes, and a spool valve element concentrically assembled with the input shaft to be shifted in response to relative rotation of the shafts in an axial direction to control the supply of fluid under pressure to the hydraulic cylinder. In use of the steering system, if defacement of the bearings for the output shaft results in an intervening space, the operator's effort in steering the steering road wheels to the right will differ from that in steering the steering road wheels to the left. For the purpose of eliminating such a defect, the output shaft is integrally provided with a sleeve-like valve casing which surrounds the spool valve element to provide reliable control of the fluid flow. It has been, however, experienced that the valve casing and the output shaft rotate jointly to cause large frictional resistance at outer peripheral sealing portions of the valve casing. As a result of the frictional resistance acting on the output shaft, smooth return operation of the steering wheel may not be expected, and the power-assisted steering operation will suffer.